Our long-term objective is a physical-chemical and molecular understanding of the processes of phototransduction in photoreceptor cells. The details of the sequence of events that mediates excitation by linking the absorption of Photons by photopigments to changes in conductance of the plasma membrane are not entirely understood. Moreover, the sequence of events that modulate the transduction of excitation to produce "adaptation" are also incompletely understood. Our specific aims include the design and performance of experiments to test current hypotheses for intracellular signalling of excitation and adaptation in both vertebrate and invertebrate photoreceptors. We propose to examine the participation of phosphoinositide metabolism in the modulation of excitatory transduction (that is mediated by changes of cGMP concentration within vertebrate rods). These assays will include determination of time course of changes in inositol polyphosphates on a rapid time scale using either rapid-freezing or microwave heating techniques, or both. We propose to investigate the candidacy of cGMP as the excitatory messenger within Limulus ventral photoreceptors by both biochemical and electrophysiological experiments. We will assay for light activation of guanylate cyclase, cGMP-phosphodiesterase, and changes in cGMP content. We will also assay the effects of pharmacological agents on these enzymes, on cGMP content and on the light responses recorded electrophysiologically. We also propose to produce "concentration jumps" of intracellular cGMP in Limulus photoreceptors by the use of "caged cGMP".